No subsequent work has examined readability so thoroughly or investigated so many style elements or the relationships between them. The authors first identified 228 elements that affect readability and grouped them under these four headings:

1. Content
2. Style
3. Format
4. Features of Organization

The authors found that content, with a slight margin over style, was most important. Third in importance was format, and almost equal to it, "features of organization," referring to the chapters, sections, headings, and paragraphs that show the organization of ideas (See Figure 4).

Fig. 4. The four major categories of readability (Gray and Leary, p. 31).

They found they could not measure content, format, or organization statistically, though many would later try (See below, "The measurement of content"). While not ignoring the other three causes, Gray and Leary concentrated on 80 variables of style, 64 of which they could reliably count. They gave several tests to about a thousand people. Each test included several passages and questions to show how well the subjects understood them.